This invention relates in general to electrophotography and, in particular, to a process for preparing electrophotographic imaging members containing a charge transport layer formed using non-chlorinated solvents.
In electrophotography, the surface of an electrophotographic plate, drum, belt or the like (hereinafter referred to simply as an imaging member) containing a photoconductive insulating layer on a conductive layer is first uniformly electrostatically charged. The imaging member is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image on the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly to a print substrate, such as paper. The imaging process may be repeated many times with reusable imaging members.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material.
In addition, the imaging member may be layered. Current layered organic imaging members have at least a substrate layer and two active layers: (1) a charge generating layer containing a light-absorbing material, and (2) a charge transport layer containing electron donor molecules.
The substrate layer may be formed from a conductive material. In addition, a conductive layer can be formed on a nonconductive substrate.
The charge generating layer is capable of photogenerating charge and injecting the photogenerated charge into the charge transport layer. U.S. Pat. No. 4,855,203 to Miyaka teaches charge generating layers comprising a resin dispersed pigment. Suitable pigments include photoconductive zinc oxide or cadmium sulfide and organic pigments such as a phthalocyanine type pigment, a polycyclic quinone type pigment, a perylene pigment, an azo type pigment and a quinacridone type pigment.
In the charge transport layer, the electron donor molecules may be in a polymer binder. In this case, the electron donor molecules provide hole or charge transport properties, while the electrically inactive polymer binder provides mechanical properties. Alternatively, the charge transport layer can be made from a charge transporting polymer such as poly(N-vinylcarbazole), polysilylene or polyether carbonate, wherein the charge transport properties are incorporated into the mechanically strong polymer.
Imaging members may also include a charge blocking layer and/or an adhesive layer between the charge generating layer and the conductive layer. In addition, imaging members may contain protective overcoatings. Further, imaging members may include layers to provide special functions such as incoherent reflection of laser light, dot patterns and/or pictorial imaging or subbing layers to provide chemical sealing and/or a smooth coating surface.
Suitable coating methods used for applying the various layers to electrophotographic imaging members include dip coating, roll coating, Meyer bar coating, bead coating, curtain flow coating and vacuum deposition. Solution coating is a preferred approach because it is more economical than vacuum coating and can be used to deposit a seamless layer.
U.S. Pat. No. 5,547,790 to Umeda et al. teaches applying charge transport layers from coating solutions comprising a charge transport material and suitable organic solvents including tetrahydrofuran, dioxane, toluene, monochlorobenzene, dichloroethane, methylene chloride and cyclohexanone.
Chlorinated solvents, such as those described above, provide charge transport layers having good coating quality. However, chlorinated solvents have environmental safety problems. As a result, the use of these solvents requires solvent abatement systems in order to eliminate leakage. Therefore, non-chlorinated solvent systems that provide coating qualities similar to or better than those of chlorinated solvent systems are desired.